Hi Cathy,
welcome to the group!!
Now, for the diet soda -- you mix it with the shake omitting the water. Don't shake it in your shaker jar. I did that without thinking one day and experienced an explosion in my kitchen.
I've found the best way for me is to first put the diet soda in my blender, add the shake and blend slowly. ( Sometimes I add crushed ice too.) That way it doesn't escape the blender!!
The "complete program" is most likely all MF and no additional food. That is, skip the "lean and green" and have 6 MF meal replacements each day. This is done with a Dr.s supervision. Individuals generally choose this when they have difficulty with portion control. The 5 and 1 program is the other choice. Five medifast meal replacements each day and a "lean and green" In my experience people lose weigh at about the same rate on either program.
I'll be watching for your progress
jan